Passivation of reactive metal surfaces



June 12, 1962 v. F. NOVY ErAL 3,038824 PASSIVATION OF' REACTIVE METALSURFACES Filed May l5, 1958 3 Sheets-Sheet 1 ...nv i

FIG.5

V/RCr/L. E NOVY CRn/G G fr/RKPAT/e/CK INVENTORS ATTORNEY `Tune 12, 1962v, F. NOVY EIAL 3,038,824

PASSIVATION 0F REACTIVE METAL SURFACES Filed May l5, 1958 I5Sheets-Sheet 2 FIGJO VlRG/L E NOVY CRA/G G. /f/HKPATRlcK INVENTORS MMML. Shu@ ATTORNEY June 12, 1962 v. F. NOvY ErAL 3,038,824

PAssIvATIoN 0F REACTIVE METAL suRFACEs Filed May l5, 1958 3 Sheets-Sheet3 FIG. I3

CRA/6 G. l K/RKPA TORS ATTORNEY rates fihce 3,038,824 PASSii/ATIN @EREAQTVE METAL SURFACES Virgil F. Novy, Altadena, and Craig G.Kirkpatrick,

Granada l-lliils, Caiif., assignors to Nuclear Corporation of America,line., New York, NPY., a corporation of Michigan Filed May l5, i958,Ser. No. 735,603 Claims. (Cl. 14S-6.14)

This invention relates to a chemical method for indefinitely passivatingthe surfaces of air reactive metals and, more particularly, thisinvention relates to a method of indefinitely passivating the surfacesof rare earth metals such as cerium, lanthnnum, praseodymium andneodymium metals, and the like.

IDuring and after the final polishing of rare earth metal samples suchas cerium, `for example, an oxide coating forms. Heretofore, before thetrue surface structure of the cerium metal could be revealed, it wasnecessary to remove this coating. There were two methods available toaccomplish this removal, i.e mechanical and chemical; the mechanicalmethod is undesirable however, due to the necessity of using a coarseabrasive to remove all traces of oxide from the low arcas and from thegrain boundaries. The use of such mechanical methods of removal leavesthe sample surface with traces of smeared metal and fine scratches. Themechanical method might possibly be utilized in an inert atmosphere or avacuum chamber, but the disadvantages of operating under such conditionsare obvious.

In accordance with the present invention, it has been found that achemical treatment of the polished metal surface, i.e. treatment of thepolished sur-face lwith an acid solution is efhcacious to passivate thesurface of active metals, such as cerium, to such an extent thatcorrosion in air is retarded for a period of time in excess of 150hours. Since the usual handling procedures for freshly prepared surfacesof air reactive metals is to dip them in heavy oil and then cover themetal surfaces with oilsoaked rags, it is apparent that the presentinvention affords many advantages over the heretofore known methods ofhandling such air reactive metals. This bulky and crude procedureformerly employed is unnecessary after the metal surface has beentreated in accordance with this invention and the treated surfaces maybe handled indetinitely in air with only reasonable care being requiredto prevent damage to the treated surface.

In the process of the invention, a freshly ground or polished surface ofa reactive metal is first cleaned with the aid of a solvent, such asethyl alcohol, to remove any traces of the original solvent used in theoriginal grinding or polishing step. The polished surface is then dippedor rubbed or swabbed for a period of about to 3() seconds in an acidsolution, such as a nital solution, nital being a mixture of 5% byweight of nitric acid and 95% by weight of ethyl alcohol. The nitalchanges the dark colored film, which forms almost immediately on thesurfaces of unpassivated metals, to a different, and usually lighter,color. This color or deposit is then removed from Ithe surface bytreating the surface with a suitable solvent such as an alcohol, forexample ethyl alcohol. This process is repeated, starting with the nital`dip or rub or swab until there is no color change detectable upontreatment with the nital. After a last `alcohol rinse, the surface isdried and the true metallic lustre is revealed. Metallic surfaces sotreated are indefinitely passivated and will not tarnish in theatmosphere at ambient conditions.

Among the solvents which may be used to clean the reactive metal surfaceto remove the original grinding solvent are alcohols such as ethyl,methyl, isopropyl, and butyl, and other compounds such as ethers, andketones, such as acetone. Among the oxidizing agents which may beemployed are mixtures of acids such as nitric, chromic,

erchloric, and pieric, with solvents such as ethyl alcohol, glycerine,glycol, and petrolatum. Other oxidizing agents such as ferrie chloride,may also be used. The acid concentration in the acid solution may be inlthe range of about 1 percent to 25 percent, preferably 5 percent to l0percent.

The treatment time with the acid solution may be in the range of about 3seconds to 60 seconds, preferably about l0 to 20 seconds. Thetemperature of the acid solution which is used lfor treatment of thepolished reactive metal surface may be in the range of about 60 to F.

Generally speaking, it is necessary to `dip or rub or swab the polishedreactive metal surface in the acid solution about l to 6 times,providing an intermediate rinse with a solvent such as alcohol, beforeno color change will be detected upon a succeeding treatment with theacid solution.

The invention will be further illustrated by reference to the followingspecific example:

Example I A `series of reactive rare earth metal samples having freshlyground or polished sur-faces were first finely polished with the aid ofethyl alcohol to remove any traces of the original solvent. Each surfacewas then dipped for a period of 20 to 30 seconds in a solution of nital(5 percent nitric acid, percent ethyl alcohol) and the dark coloredoxide film, which forms almost immediately on the surfaces ofunpassivated metals when exposed to air, was changed to a different and`lighter color. This color was removed from the surfaces by swabbingwith ethyl alcohol and the nital `dip and alcohol treatment wererepeated 3 times, i.e. until no color change was detectable upontreatment with the nital. After rinsing each of the surfaces with ethylalcohol, the surfaces were dried and the true metallic lustre wasrevealed.

in the accompanying drawings, a series of photomic-rographs are shown,depicting both unpassivated and passivated reactive metal surfaces, thepassivation treatments being conducted as above described.

iFIGURE l is a photomicrograph of a cerium metal sample showing apolished surface which had been exposed to air at room temperature (75to 85 F.) for a period of hours. The print shows the corrosion filmformed on lthe surface of the metal. Further exposure to air resulted inprogressive corrosion.

'FIGURE 2 is a photomicrograph of a sample of the same cerium metalshown in FIGURE 1 passivated in accordance with this invention. In thepassivating treatment, the corrosion film which had formed during theshort interval of ltime between polishing and passivation (l to 5minutes) was removed and the surface of the metal was stabilized asshown.

FIGURE 3 shows the same surface of the cerium metal sample shown inFIGURE 2 after the metal surface had been exposed to air at roomtemperature (75 to 85 F.) for a period of 45 days. Microscopicexamination of the sample, after exposure to air at room temperature fora period of 60 days, revealed the same structure with the absence of acorrosion film, indicating that the `sample would remain in this stablecondition indefinitely.

FIGURE 4 is a photomicrograph of another sample of cerium metal showinga polished surface thereof. The print lshows the corrosion film whichformed on the metal surface after exposure to air for a period of only lto 3 minutes.

IFIGURE shows the same metal surface shown in FIGURE 4 after passivationin accordance with this invention. This passivated surface was studiedmicroscopically after exposure to air at room temperature for a periodof hours and no change in the structure could be ascertained. Theabsence of any additional corrosion indicated that the metal surfacewould remain stable indefinitely.

FIGURE 6 shows a typical corrosion film `formed on the surface oflanthanum metal.

FIGURE 7 shows a typical corrosion lm formed on the surface ofpraseodymium metal, and FIGURE 8 shows a typical corrosion Ifilm formedon neodymium metal.

Typical passivated surfaces of the metals shown in FIGURES 6, 7 and 8are shown in yFIGURES 9, 10 and 11, which are passivated surfaces oflanthanum, praseodymium and neodymium respectively.

LFIGURE 12 shows the amount of intergranular corrosion which occurredduring a 20 hour exposure to air at room temperature of the passivatedsurfaces of the lanthanum metal. An unpassivated surface of lanthanummetal exposed to air at room temperature for the same length of timeunder the same conditions was completely corroded and was impossible tophotograph due to the corrosion powder or residue which completelycovered the surface.

FIGURE 13 shows the pit-type of corrosion which occurred during a 20hour exposure to air at room temperature on the passivated surface ofneodymium metal. An unpassivated surface of the neodymium metal exposedto air at room temperature for the same length of time and under thesame conditions was also impossible to photograph as the corrosion filmcompletely obscured the grain boundaries shown in FIGURE 8.

Both samples of passivated cerium metal, s hown in FIGURE 5, andpassivated praseodymium metal, shown in FIGURE 10, were unaffected afterexposure to air at room temperature for 20 hours, under identicalconditions.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A method of passivating the polished surface of a rare earth metalwhich comprises treating the polished surface with a solvent capable ofremoving therefrom all traces of material present on said surface afterthe polishing operation to remove said material from the surface,apply-ing an oxidizing agent to the thus treated surface,

and removing the deposit `formed by the oxidizing agent, by treatingsaid surface with a solvent for said deposit, and drying the surface.

2. A method according to claim 1 in which the solvent is an organicsolvent.

3. A rare earth metal having a lustrous polished surface passivatedaccording to the method of claim 1 to render the metal of the surfacemore resistant to corrosion in air than natural polished metal, wherebysaid polished surface retains its natural lustre after polishing.

4. A method according to claim 2 in which the solvent is an alcohol.

5. A method according to claim 1 in which the oxidizing agent is nitricacid.

6. A method according to claim 1 in which the oxidizing agent is nitricacid in admixture with an alcohol in the proportions of about 1% to 25%of the acid and the balance the alcohol.

7. A method according to claim 6 in which the alcohol is ethyl alcohol.

8. A method of passivating the polished surface of a rare earth metallwhich comprises treating the polished surface with an alcohol to removefrom said surface all traces of material present thereon after thepolishing operation, applying to the thus treated surface an oxidizingagent comprising a mixture of nitric acid and alcohol, again treatingthe surface with an alcohol to remove therefrom the deposit formed bythe oxidizing agent, and drying the surface.

9. A method according to claim 8 in which the alcohol is ethyl alcohol.

10. A rare earth metal having a lustrous polished surface which ispassivated according to the method of claim 8 to render the metal of thesurface resistant to corrosion in air, whereby to retain its metalliclustre after polishing.

References Cited in the tile of this patent UNITED STATES PATENTS2,408,155 Thornbury Sept. 24, 1946 2,523,892 Warf Sept. 26, 19502,777,785 Schuster et al. Ian. 15, 1957 FOREIGN PATENTS 103,593Australia Mar. 29, 1938 OTHER REFERENCES Metals Handbook, 1948 edition,page 394. Levy: The Rare Earths, p. 126, par. 1, 2nd ed., Longmans,Greene & Co., 1924.

1. A METHOD OF PASSIVATING THE POLISHED SURFACE OF A RARE EARTH METALWHICH COMPRISES TREATING THE POLISHED SURFACE WITH A SOLVENT CAPABLE OFREMOVING THEREFROM ALL TRACES OF MATERIAL PRESENT ON SAID SURFACE AFTERTHE POLISHING OPERATION TO REMOVE SAID MATERIAL FROM THE SURFACE,APPLYING AN OXIDIZING AGENT TO THE THUS TREATED SURFACE, AND REMOVINGTHE DEPOSIT FORMED BY THE OXIDISING AGENT, BY TREATING SAID SURFACE WITHA SOLVENT FOR SAID DEPOSIT, AND DRYING THE SURFACE.